Peel and grate the potatoes on the thick side of the grater, place grated potato in a clean tea towel and squeeze out as much liquid as you can, by twisting the towel.
Melt 20g/1⅓ tbsp butter and add it to the grated potato with a good helping of ground pepper and a tsp of salt, mix well.
Heat a 26cm/10in non-stick frying pan to a medium heat, add a splash of oil and then 20g of butter, once the butter is bubbling add the potato, spread it out and tidy the edges. Don't compact the potato.
Heat a separate frying pan to med high, add a splash of oil and fry the chorizo for 1-2 mins on each side and remove from the pan.
After about 10 minutes lift to check crispiness, if happy lift out onto a board with two spatulas or if you're confident turn in onto a board or plate (you can just make smaller rostis, which makes the whole turning thing a lot easier).
Melt 20g butter in the pan and add back the rosti to cook for anther 10 mins (or to happiness).
Just before the rosti is turned, heat the chorizo pan to a medium heat, add 20g butter (there still should be some residual oil from cooking the chorizo) and add the onions, fry for a couple of minutes and add the garlic, cook for a further minute and add the mushrooms, a good grind of pepper, half a tsp of salt and the chilli flakes, stir and cook for about 3-4 minutes until the mushrooms are browned.
Increase the heat to high and add the white wine to deglaze the pan, bring to a boil and reduce the wine by half, then add the cream, reduce the heat to medium stirring regularly so that it doesn't catch.
Check the rosti, remove from the pan onto a board and slice to serve.
Reduce the cream for 2-3 minutes and add the parmesan, stir in, once you have the desired consistency of the sauce add the parsley and serve on a slice of the rosti.
